Current sources are used in a variety of applications. An ideal current source has infinite source impedance and is insensitive to the voltage present at its source terminal. An ideal current sink behaves similarly, i.e., the magnitude of current drawn by the sink terminal is insensitive to the voltage present on the sink terminal.
Although practical current sources deviate from ideal behavior, current sources find wide use in a range of circuit applications and practical current sources having good real-world behavior can be constructed. While current sources may be implemented using relatively simple circuitry, more complex circuitry is typically used for more sophisticated application, such as in the implementation of so called floating current sources.
For example, certain types of sensors operate as variable resistors and require a bias voltage across their resistor terminals in order to operate properly. Similarly, some controllable resistors also require a bias voltage across the controllable resistor pins. Because a true floating current source presents high impedance to both pins of the resistor being biased, it is possible to use it to bias variable or controllable resistors in applications where both pins of the resistor must appear to float with respect to the bias network.
In some applications, it is also useful to float the resistor at some known DC voltage with respect to circuitry used to vary the resistance of a controllable resistor or circuitry used to detect the resistance of a variable resistor, while still presenting high AC impedance to both pins of the resistor being biased. Some known circuits are referred to as floating current sources although they do not truly “float,” because one terminal exhibits low impedance with respect to some voltage source, e.g., ground or power. In other instances, circuits referred to as floating current sources in reality operate as floating current sinks and require some minimum external voltage across the current sink terminals.
Further, while true floating current sources are known, such circuits generally use multiple operational amplifiers and/or combinations of several transistors and supporting circuitry, which circuitry is comparatively complex as compared to the teachings presented herein. Such complexity leads to undesirable cost and, in some cases, excessive component count and/or consumption of limited circuit board area.